In known systems of this kind, when the hydraulic or pneumatic pressure of the working fluid supplied to the actuating jack fails for any reason, then, while this jack is set so that the supercharging air is being passed to the cooling means, the jack is no longer able to work against the action of the resilient actuating element (typically a return spring). The latter therefore returns the obturator to its first position, bypassing the cooling means. As a result uncooled supercharging air is fed to the engine, which entails a danger of damage to the engine.
In addition, it is desirable that the supercharging air should not penetrate into the engine at too high a pressure. For this reason, it is possible to take the air through the cooling means so that it loses some of its energy, even when it is no longer necessary to cool it.